HMV 801 radiogram.

[FERNSEH]

My HMV 801 radiogram was donated to me by a forum member three years ago.
The 801 is a truly huge piece of equipment, it must weight 150 pounds or more!
The 801 radiogram was made in 1937 and was the successor to the model 800 of 1936.
The original push-pull output valves were a pair of Marconi PX25s, it goes without saying that there is no chance of finding those at a reasonable price.
The best replacement for the PX25 is without doubt a triode connected KT66, this valve will work OK on the 450 volt HT supply.
Purists may not like this, but I intend to replace the original B4 valveholders with octal sockets for the replacement valves. I'm not going to bother with valve socket adaptor units.
The characteristics of a triode connected KT66 operating on a 400 volt HT supply is very similar to the PX25 triode.
The original HT rectifier valves are a pair of Marconi U14s, these are directly heated valves so it might be a good idea to introduce an HT delay arrangement.
The KT66s will take at least 60 seconds to warm up, whereas the directly heated triodes would have reached their full emission within a few seconds.

I might replace the U14s with a pair of silicon diodes, something that again might cause the purists displeasure.

Look forward to your comments.

DFWB.

[Leon Crampin]

I would deprocate making "structural" changes to such old equipment - the right spares often turn up eventally and the Internet is your means of getting them.

Put your revised output valves on adapters for the time being (I don't know what you propose in terms of heater supply arrangements) and use B4 base indirectly heated rectifiers such as the MU14 - easily available. Don't put any silicon devices near it.

I'm not a great fan of HMV equipment but it certainly has its place in history. It ought surely to be repaired in such a way as to be usable in the short term (wrong valves fitted on adapters etc.) and restored as and when the correct parts surface, by you or your sucessor. If you remove original bits, they always get lost.

Leon.

[Heatercathodeshort]

I would say certainly go ahead and fit the more easily and cheaply available KT66 output valves. The performance will be the same and they are a very good match to the PX25 when connected as a triode. There is nothing special about PX25's as such. They are a simple low frequency output triode, nothing magical as some seem to think. A very simple valve to make if you were a valve manufacturer. Must admit I would agree with Leon here and probably fit them using old B4 bases from U/S valves. Let me know how you get on with it David as I intend to fit KT66's to my 800 gram as its used quite a lot.
I have plenty of spare 66's, some new, some good s/h so will have enough to see me out!
My PX4's and 25's are rather thin in the stores and don't want to reduce it further with accidents! The 66 connected as a triode with around 450 on the anode will require around 600 ohms bias resisitor. I think the one in the 801 [same amp as 800] is within 5% of that value. Hope it works out. Regards, John.

[Peter.N.]

You will require a 6.3 volt heater supply for the KT66's as your general heater supply is 4 volts.

[FERNSEH]

Originally Posted by Heatercathodeshort

The 66 connected as a triode with around 450 on the anode will require around 600 ohms bias resisitor. I think the one in the 801 [same amp as 800] is within 5% of that value. Hope it works out. Regards, John.

Hi John,
That's right. The published data for a triode connected KT66 indicates that with a plate voltage of 450 volts the cathode bias resistor should be 615 ohms. In the 801 the bias resistors for the PX25s are 600 ohms, so that's near enough.
Concerning the rectifier valves. The U14s/MU14s could be replaced by indirectly heated types. This will provide the HT supply delay during the warm-up time of the KT66s.
I did consider other substitute valves. There is the 807 which has characteristics close to the 6L6. I have just received today from Langrex a pair of 5B254s. This Brimar/STC/ITT made valve is similar to the 807 but has a more compact glass bulb and uses the B8G loctal base.
The KT66 pair also came from Langrex.

DFWB.

[FERNSEH]

Originally Posted by Peter.N.

You will require a 6.3 volt heater supply for the KT66's as your general heater supply is 4 volts.

Hi Peter,
That's not a problem because each PX25 filament is supplied by separate 4 volt windings. I can wire them in series and insert a resistor in series with each KT66 heater to obtain the correct 6.3 volts.
The heater current of the KT66 is 1.3 amps. The PX25 has a 4 volt 2 amp filament.

DFWB.

[AlanBeckett]

Errrrr.
Am I doing something dumb here?
Putting the heater windings in series gives you 8 volts at 2 Amps.
Two KT66s require 2.6 Amps.
Alan

[FERNSEH]

Originally Posted by AlanBeckett

Errrrr.
Am I doing something dumb here?
Putting the heater windings in series gives you 8 volts at 2 Amps.
Two KT66s require 2.6 Amps.
Alan

I know of others who have fitted EL34s into 800 and 801 radiograms without any problems. The EL34 has a 1.3 amp heater. I'm sure the mains transformer can tolerate the overload.
The plan is to get the 801 going first, see how it performs with the substitute output valves and perhaps later on fit a 6.3 volt heater transformer.

The 5B254 beam tetrodes have 0.9 amp heaters and are OK at 400 volts HT.
I could try those first.

DFWB.

[Heatercathodeshort]

Hello again David,
I certainly think this is a worth while project as you can easily return the gram to original condition. Its not as if your bolting AD161/162 matched pairs to the rear panel.. I suspect that the original transformer is well capable of supplying the extra current. Its unlikely that EMI would get it spot on at 2 amps. They usually favoured the belt and braces approach so its worth a trial run.
I have just gone through my stock of mains transformers and found several of these Metropolitan Vickers heater transformers. At first sight they appear to have been fitted to some device that you would install in your garden to deter cats but closer inspection shows that they must have been used in a kinder sense.
The primary is 230v and there are three 6.3v secondaries, 2amps, .3amp and .4amp. so if you are having difficulty locating a transformer you are more than welcome to relieve me of one of these!
Regards, John.
P.S. It measures 10cm X 14cm including feet.

[murphyv310]

Hello David.
This is a very good mod to do on your 801.
I particularly like the KT66 and its bigger brother the KT88.
As yourself & John say the PX25 is really just a run of the mill O/P triode.
I have a couple of spare MU14's here and a really meaty mains transformer with two of each seperate 4v, 5v, & 6.3v windings, also HT windings of 450/500-0-450/500v @ 150ma. The 6.3v windings are 5 amp each, It weighs a ton though.
I modified my RGD a few years ago and fitted a Quad 22 preamp and David Bogen amp, as you know it performs a treat but with 21 valves its a grand heater.
Good luck with the 801.
Cheers
Trevor

[FERNSEH]

Still no decision on what valves to employ in the output stage.

The tube manufacturing firm called KR is located in the Czech Republic, they make an excellent copy of the original PX25 along with other power triodes. The valves are very good but I believe they don't come cheap. It will be worthwhile making some enquiries to find out if KR market their valves in the UK.
The radiogram works well with PX4 valves, but with over 400 volts supplied to their anodes they are working way outside the safe limits for this type of valve. For the PX4 VA max is 250 volts.
Meanwhile, I've started work on the cabinet. On Saturday evening I stripped off the original lacquer from the cabinet top. The whole cabinet will need attention.

DFWB.

[Neil F]

Rod Burman certainly handles KR valves: even though their PX25s aren't specifically listed on his website, it would be worth contacting him to see what's what.

http://www.valves.uk.com/


(No connection, etc.)

[Heatercathodeshort]

Nice start David,
The veneer is more like 3 ply so it will take a good rub down! I ran that gram with PX4's for over 10 years playing 78's at the rear of my old shop. I know the purists will throw up their hands in horror and will require councelling at my admission but they ran smoothly with no overheating or distortion. That same pair are at present belting out 10 watts in an amplifier in my 'Den'. My own personal thoughts are that the two valves are somewhat similar. There is only so much you can do with an audio power triode with three electrodes and I think all these early valves were built to a very wide tolerance. Only my views of course discovered from hands on experience but as you have probably guessed I would probably have been pushed off the M.O.V. factory roof if I had suggested that 40 years ago....Good luck with that gram. I have two boxes of 78's for you awaiting its completion. Regards, John.

[PJL]

The peak DC anode volts has always seemed a bit of an odd specification as the valves have the AC signal swing on top of the DC anode voltage.

[chipp1968]

Am I correct in believeing a version of this gram was available up untill about 1950?

[Herald1360]

Interestingly, although the early PX4 is only rated to 250V Va and later ones to 300V, the data sheets give curves with higher anode voltages applied (albeit dotted lines).

It would seem that running them like this would call for higher value cathode bias resistors, as to stay within dissipation limits at 400V with a later PX4 will require about -65V grid bias (early one about -70V).

The PX25 requires lower bias voltages

http://frank.pocnet.net/sheets/060/p/PX4.pdf

http://frank.pocnet.net/sheets/126/p/PX4.pdf

http://frank.pocnet.net/sheets/060/p/PX25.pdf

Chris

[FERNSEH]

Originally Posted by chipp1968

Am I correct in believeing a version of this gram was available up untill about 1950?

Hi Chipp,
Somehow I think you are right. I believe that HMV did later on market a radiogram featuring the same styling cues.

The PX4 was introduced into the market place in 1929 and was designed to replace the earlier LS5 series of valves. The early PX4 had a 0.6amp filament and had modest output power of 1.1 watts. In 1931 the valve was uprated and the filament current became 1 amp. It received further upgrades thoughout it's production life and the late versions could take a plate voltage of 300 volts.
The PX25 is said to be the successor to the LS6A.

DFWB.